1. Field of the Invention
This invention relates to mounting mechanisms for zoom lenses and more particularly to mounting mechanisms for zoom lenses of the type in which a diaphragm is arranged between the variator and compensator.
2. Description of the Related Art:
For the zoom lens shown in FIG. 1(a) comprising a focusing lens L.sub.1, a variator lens L.sub.2, a compensator lens L.sub.3 and an image forming lens L.sub.4, this conventional type of mounting mechanism is constructed in such a manner that a focusing lens holder 1 containing the lens L.sub.1 is helicoid-threadedly mounted on a body tube 2 upon rotation to move the focusing lens L.sub.1 axially.
Also, the variator lens L.sub.2 and the compensator lens L.sub.3 are held in lens holders 3 and 4 respectively, and these lens holders 3 and 4 are arranged to axially move as guided by the known 3-bar method or guide bars 5. The axial movement of the variator L.sub.2 and compensator L.sub.3 is controlled by cam lifts 6a and 6b provided in a cam sleeve 6 rotatably fitted in the body tube 2. A zoom ring 7 is rotatably fitted on the body tube 2 and is drivingly connected to the cam sleeve 6 through a pin 8 planted on the cam sleeve 6. Also, the image forming lens L.sub.4 then follows after a diaphragm 9 and its holder 10 is fixedly mounted in a rear fixed barrel 11 by screw fasteners 12.
The cam sleeve 6 is restrained from axial movement by front and rear mask plates 13 and 14 which bear the three guide bars 5.
In the above-described construction of the zoom lens mounting mechanism, it has been customary that the cam sleeve 6 is elongated so as to cover the entire space in which both variator L.sub.2 and compensator L.sub.3 move. In application to, for example, a zoom lens optical system having the diaphragm 9 in that space, therefore, the diameter of the outer lens barrel becomes very large (bulky) and the structure of the mounting mechanism becomes complicated, producing problems that the difficulty of assembly and the number of parts increase with increase in the production cost. Further, when applied to another optical system as shown in FIG. 1(a), because the diaphragm position is far away from the focusing lens, for the angular field is increased, the focusing lens becomes bulky, making the outer appearance of the zoom lens mounting awkward, and its weight very heavy.
In respect to the weight, the first component of the zoom lens accounts for 5 to 8 tenths of the entire body thereof. To reduce the weight, it is, therefore, required that materials of smaller specific gravity be employed, or the volume be minimized. Because such materials have a little freedom of choice, in order to achieve a desired reduction of the weight in such a manner as to preserve high grade of imagery, it is of great importance how to reduce the diameter of the front component. That is, as the volume may be considered to vary in proportion to the cube of the diameter, when the diameter can be reduced 10%, it becomes possible to reduce the volume by 27%. By the way, the diameter of the front component has to be determined by the larger of the diameters of the oblique light bundle in the wide angle end and the paraxial light bundle in the telephoto end. So, it becomes necessary to reduce both of them.
To eliminate this problem on optics, an attempt has been made by arranging the diaphragm in between the variator and compensator in Japanese Laid-Open patent application No. SHO 59-13212. With the optical system comprising, from front to rear, the focusing lens, variator, diaphragm, compensator and relay lens, even if the angle of field is widened, due to the shortened distance of the diaphragm to the focusing lens, there is a relatively large room of freedom of the oblique light bundle, thus permitting the focusing lens to be designed depending on the axial light bundle in the telephoto end. Therefore, by making the F-number in the telephoto end larger than that in the wide angle end, the focusing lens can be remarkably reduced as compared with the first optical system.
Another feature of the conventional lens arrangement of FIG. 1(a) is that the diaphragm is constructed as shown in FIG. 1(b) with a plurality of blades 9b.sub.1, 9b.sub.2, . . . , 9bn pivotally mounted on a casing 9a and operates with a galvanometer drivingly connected through a crank lever 9c to a control lever 9d for the blades. And, the casing 9a is fixedly secured to the inner surface of the rear fixed barrel 11 by screw fasteners.
Because the bore of the outer fixed barrel 11 is round in shape, the casing 9a and the arrangement of the blades 9b must be conformed to that round shape.
For the present invention is applied to the mounting mechanism for the lens optical system, the system comprising, from front to rear, the focusing lens, the variator lens, the diaphragm, the compensator lens and the relay lens, which mechanism has the guide bars to suspend the variator and compensator lenses, the arrangement of such diaphragm unit in between the variator and compensator leads to an extraordinary increase of the diameter of the outer barrel.